The present invention relates to protection diodes. Protection diodes are here diodes that clip the voltage across their terminals as soon as the latter reaches a predetermined value, called breakdown voltage, of any polarity. Such diodes are commonly used at the input of electronic devices liable to receive spurious pulses, such as circuits connected to telephone lines. In practice, the breakdown voltage of these diodes must be accurately determined (usually from 5 to 10%) in order not to impair the circuit they are intended to protect.
The invention more particularly relates to the case when it is desired to protect first lines from overvoltages of a first level and second lines from overvoltages of a second level. This is schematically illustrated in FIG. 1 in which a first set of conductors L11, L12 . . . and a second set of conductors L21, L22 . . . are represented. It is desired to protect the first set of conductors with respect to a common terminal C by first bidirectional breakdown diodes D11, D12 . . . and the second set of conductors by second bidirectional breakdown diodes D21, D22 . . . Diodes D11, D12 . . . of the first set break down when the voltage across their terminals exceeds in absolute value a voltage V1 and diodes D21, D22 . . . of the second set break down when the voltage across their terminals exceeds in absolute value a voltage V2. In the following it is assumed that V2 is higher than V1.
In the prior art, to provide a protection of the type required in FIG. 1, discrete protection diodes or monolithic circuits, each incorporating a set of protection diodes having a predetermined breakdown voltage, have been used. In both cases, long and sometimes complex assembling is to be achieved.
Thus, an object of the invention is to provide one monolithic integrated structure grouping two sets of protection diodes, each of these sets having distinct breakdown characteristics.
A further object of the invention is to provide such a structure which is simple to manufacture for predetermined couples of breakdown voltages.